Correlations among indices of electric and magnetic field exposure in electric utility workers

Abstract

Power-frequency electric and magnetic fields are known to exhibit marked temporal variation, yet in the absence of clear biological indications, the most appropriate summary indices for use in epidemiologic studies are unknown. In order to assess the statistical patterns among candidate indices, data on 4383 worker-days for magnetic fields and 2082 worker-days for electric fields collected for the Electric and Magnetic Field Project for Electric Utilities using the EMDEX meter [Bracken (1990): Palo Alto, CA: Electric Power Research Institute] were analyzed. We examined correlations at the individual and job title group levels among indices of exposure to both electric and magnetic fields, including the arithmetic mean, geometric mean, median, 20th and 90th percentiles, time above lower cutoffs of 20 V/m and 0.2 microT, and time above higher cutoffs of 100 V/m and 2.0 microT. For both electric and magnetic fields, the arithmetic mean was highly correlated with the 90th percentile; moderately correlated with the geometric mean, median, and lower and higher cutoff scores; and weakly correlated with the 20th percentile. Electric and magnetic field indices were generally weakly correlated with one another. Rank-order correlation coefficients were consistently greater than product-moment correlation coefficients. Job title group summary scores showed higher correlations among electric field indices and magnetic field indices and between electric and magnetic field indices than was found for individual worker-days, with only the 20th percentile clearly independent of the others. These results suggest that individuals' exposures are adequately characterized by a measure of central tendency for electric and magnetic fields, such as the arithmetic or geometric mean, and an indicator of a lower threshold or cutoff for each field type, such as the 20th percentile or proportion of time above 20 V/m or 0.2 microT. A single measure of central tendency for each type of field appears to be adequate when exposures are assessed at the job title level.

Exposure of welders and other metal workers to ELF magnetic fields

“…The highest rank-order correlation coefficient between the summary measures for the metal workers was found between GM and MD (0.96). Exactly the same value has been found for a very large group of electric power utility workers [Savitz et al, 1994]. Very high correlations were found between P 90 and F 0.2 (0.93), P 99 and F 1.0 (0.95).…”

Assessment and grouping of occupational magnetic field exposure in five electric utility companies

“…Using the TWA alone did not sacrifice statistical power in our study of electrical utility workers (20). However, the correlation of the TWA magnetic fields with lower cut-off scores, electric fields, and high-frequency transients (27,28) were generally weak and inay need to be assessed separately.…”

“…The job-exposure matrix features only one exposure measure (TWA magnetic field), but several other potential measures, like the geometric mean, median, 90th percentile, and higher cut-off scores correlate reasonably well with the TWA (27,28). Using the TWA alone did not sacrifice statistical power in our study of electrical utility workers (20).…”

Exposures of children in Canada to 60-Hz magnetic and electric fields

“…Examinations of the relationships of alternative indices of magnetic and electric field exposures among electric utility workers (24)(25)(26) indicate that the use of the arithmetic and geometric means at the level of job title generally sacrifices little information on most other indices. For the general population there are, to our knowledge, no published reports of correlations between various exposure indices.…”